WASHINGTON -- Federal safety officials yesterday attributed the midair collision that killed Sen. John Heinz of Pennsylvania and six others in April to poor judgment by the pilots of the two aircraft involved.

The pilot of the chartered plane that was carrying Senator Heinz toPhiladelphia International Airport should never have let a helicopter fly nearby in an attempt to check for a possible malfunction of the plane's nose wheel, the National Transportation Safety Board concluded.

And the helicopter pilot should never have tried such a dangerous maneuver over a crowded residential area, the board said.

Falling debris from the crash, which occurred on April 4 in Lower Merion Township, a Philadelphia suburb, killed two children in a schoolyard. Also killed were the two pilots of the plane, a two-engine Piper Aerostar PA-60, and the pilot and co-pilot of the helicopter, a Bell 412.

Safety board members described the Piper Aerostar's captain, Trond M. Stegen of Hughesville, Pa., as poorly trained and inexperienced. The board found that he had not conducted a simple test that would have told him whether the nose wheel was set correctly.

"This guy did not know the airplane at all," said John K. Lauber, a board member. "The bottom line is, he was not trained and checked correctly."

The board concluded that the poor training program offered by the company that operated the plane, Lycoming Air Taxi of Montoursville, Pa., was partly to blame for the accident.

Investigators said the accident occurred because a light in the cockpit failed to indicate that the nose wheel was locked in the down position for a landing. The pilot agreed to a midair rendezvous with the helicopter, whose pilots offered to take a close look at the landing gear.